Linear resonant actuator as a tap, touch and pressure sensor using back EMF

ABSTRACT

The present disclosure provides systems and methods for using a linear resonant actuator (“LRA”) to determine a type of contact between a device and its surroundings. The LRA may be coupled to an amplifier by one or more switches. The audio amplifier may receive a signal from a microcontroller and transmit the signal the LRA when the switches are closed. When the switches are in an open position, the LRA may be actively sensing for the type of contact. The back EMF may be measured when the switches are open. The measured back EMF waveform may be used to determine the type of contact. When the signal is not being transmitted, the LRA may be passively sensing to determine whether the device was tapped.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/136,811, filed on Dec. 29, 2020, the disclosure of which isincorporated herein by reference.

BACKGROUND

Mobile and wearable devices, such as smart phones and smart watches,include a linear resonant actuator (“LRA”) to provide haptic feedback toa user. The LRA is a linear electric motor that uses resonance toprovide haptic feedback. For example, the LRA may produce a vibrationwhen the device is touched by a user. However, a separate component orsensor is used to determine whether the device was touched by the userand/or the pressure of the touch applied to the device by the user. Thismay increase the costs and complexity of manufacturing the device.

BRIEF SUMMARY

The present disclosure generally relates to using an LRA to determine atype of contact with the device. For example, the type of contact may bea user holding the device, the device resting on a material surface, thedevice being worn by a user, or a touch. The types of contact betweenthe device and its surroundings may have different characteristics, suchas a speed of contact with the device, a pressure of contact with thedevice, a duration of contact with the device, a material in contactwith the device, or a surface area in contact with the device. The LRAmay be coupled to an audio amplifier by one or more switches. The audioamplifier may receive a signal from a microcontroller. The audioamplifier may transmit the signal to the LRA when the switches are in aclosed position. When the switches are in an open position, a backelectromotive force (“EMF”) of the LRA may be measured or determined.For example, an instrumental amplifier may be used to determine, orcapture, the back EMF. The back EMF may be measured in volts (“V”).

Depending on the amount of pressure of the touch, and/or what istouching the device will determine how much dampening there is to theLRA and, therefore, how much the back EMF drops. For example, the humanbody is a soft viscoelastic material that can absorb energy depending onthe contact force. The human body may absorb some of the energy from theoscillation of the mass of the LRA and, therefore, dampen theoscillation. Dampening the oscillation of the mass of the LRA may reducethe amount of back EMF.

The determined back EMF may be compared to a previously determined backEMF, such as the back EMF detected during a previous reading. Forexample, the determined back EMF may be compared to a back EMF that wasdetermined 10, 12, 15, 25, etc. readings ago. If the difference betweenthe determined back EMF and the back EMF from any number of readings agois higher than a threshold, it may be determined that the device wastouched. In some examples, determining the pressure of a touch on thedevice may be determined by comparing the determined back EMF to abaseline threshold. The pressure may be the difference between thedetermined back EMF and the baseline threshold.

One aspect of the disclosure includes a device comprising a linearresonant actuator (“LRA”) and one or more processors in communicationwith the LRA. The one or more processors may be configured to measure aback electromotive force (“EMF”) of the LRA and determine, based on themeasured back EMF, a type of contact between the device and itssurroundings. The one or more processors may be further configured tomeasure a damping of an internally moving mass. The LRA may include theinternally moving mass, a magnet coupled to the internally moving mass,and a spring, wherein the internally moving mass and the magnet arecoupled to the spring. The device may further comprise a comparatorconfigured to transmit a trigger based on the measured back EMF.

The type of contact may include a user holding the device, the deviceresting on a material surface, the device being worn by a user, or atouch. The type of contact between the device and its surroundings maybe based on at least one of a speed of contact with the device, apressure of contact with the device, a duration of contact with thedevice, a material in contact with the device, or a surface area incontact with the device. When determining the type of contact is thetouch, the one or more processors may be further configured to comparethe measured back EMF to a previously measured back EMF. Whendetermining that the type of contact is the touch, the differencebetween the determined back EMF and the previously measured back EMF maybe greater than a predetermined threshold. When determining the pressureof contact with the device, the one or more processors may be furtherconfigured to determine a difference between the measured back EMF and abaseline threshold.

The device may further comprise an amplifier and one or more switchescoupling the amplifier and the LRA. The one or more processors may befurther configured to transmit a signal to the amplifier and switch theone or more switches from a closed position to an open position or fromthe open position to the closed position, wherein when the one or moreswitches are in the open position the signal is not transmitted from theamplifier to the LRA. Determining the back EMF of the LRA may furthercomprise determining the back EMF when the one or more switches are inthe open position. When the one or more switches are in the closedposition, the LRA may produce oscillating back EMF voltage.

Another aspect of the disclosure includes a method, comprisingmeasuring, by one or more processors, a back electromotive force (“EMF”)of a linear resonant actuator (“LRA”) and determining, by the one ormore processors based on the measured back EMF, a type of contactbetween a device and its surroundings.

Yet another aspect of the disclosure includes a device comprising alinear resonant actuator (“LRA”) including a mass, a magnet, and aspring, a comparator, and one or more processors in communication withthe LRA and the comparator. The one or more processors may be configuredto detect a disturbance of the mass and the magnet on the spring andmeasure a back electromotive force (“EMF”) based on the detecteddisturbance, wherein the comparator is configured to transmit a triggerbased on the measured back EMF. The trigger may cause the device totransition from a sleep state to an awake state. The disturbance may bean external force exerted on the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a functional block diagram of a device in accordance withaspects of the disclosure.

FIG. 1B is a schematic drawing of a back EMF measurement circuit of FIG.1A in accordance with aspects of the disclosure.

FIG. 1C is a mechanical representation of a LRA in accordance withaspects of the disclosure.

FIG. 1D is a schematic drawing of a LRA in accordance with aspects ofthe disclosure.

FIG. 2 is an example signal according to aspects of the disclosure.

FIG. 3 is a graphical representation of the acceleration and back EMF ofthe LRA in accordance with aspects of the disclosure.

FIG. 4 is a graphical representation of back EMF when a device is beingtouched in accordance with aspects of the disclosure.

FIG. 5 is another graphical representation of back EMF when a device isbeing touched in accordance with aspects of the disclosure.

FIG. 6 is a schematic diagram of a sensing circuit in accordance withaspects of the disclosure.

FIG. 7 is an example graphical representation of back EMF produced whentapping a device in accordance with aspects of the disclosure.

FIGS. 8A and 8B are pictorial diagrams illustrating example uses of theLRA in accordance with aspects of the disclosure.

FIGS. 9A-9C are pictorial diagrams illustrating further example uses ofthe LRA in accordance with aspects of the disclosure.

FIG. 10 is a flow diagram illustrating a method of determining a type ofcontact with a device in accordance with aspects of the disclosure.

DETAILED DESCRIPTION

A device, such as a mobile or wearable device may use a linear resonantactuator (“LRA”) that is already within the device to determine a typeof contact between the device and its surroundings. Wearable and mobiledevices may include, for example, smartwatches, audio visual and/orvirtual reality headsets, smart phones, tablets, etc.

The type of contact may include a user holding the device, the deviceresting on a material surface, the device being worn by a user, or afingertip touch. The type of contact may be based on at least one of aspeed of contact with the device, a pressure of contact with the device,a duration of contact with the device, a material in contact with thedevice, or a surface area in contact with the device. In some examples,the fingertip touch may be an instantaneous touch, such as a tap of thefingertip on the device, or an extended touch, such as pressing a buttonon a device. An instantaneous touch or tap may have a higher velocitythan an extended touch. In some examples, the instantaneous touch mayhave a shorter duration than an extended touch. According to someexamples, the fingertip touch may have a shorter duration as compared toa user holding the device or the device resting on the material surface.However, a fingertip touch is only one example of a type of touch and isnot intended to be limiting. The pressure of contact with the device mayremain constant when the device is being worn by a user, such as when auser is wearing a smartwatch, whereas the pressure of contact with thedevice may vary as the device is being held by a user.

The LRA within the device may include a mass attached to a spring suchthat the mass oscillates along an axis in line with a voice coil, amagnet coupled to the mass, and a voice coil that provides a voltagethereby causing the mass and magnet to oscillate. The voltage providedby the voice coil may cause the magnet coupled to the mass to oscillate.The motion of the oscillating mass may produce a counter-electromotiveforce, or back EMF.

The resonant frequency of the LRA may be determined by the springconstant and the weight of the oscillating mass. According to someexamples, the LRA may have a resonant frequency between 150-200 Hz, ashuman skin perception is sensitive in that range. A driving electricalalternating current (“AC”) signal may be supplied at the resonantfrequency. The AC signal may cause the mass to oscillate at the resonantfrequency. In some examples, the LRA may be driven at lower frequenciesto avoid resonance, minimizing perception and noise. For example, theLRA may be driven at approximately 50 Hz. However, in some examples, theLRA may be driven at 53 Hz, 60 Hz, 42 Hz, etc. Therefore, driving theLRA at 50 Hz is only one example and is not intended to be limiting.

An electrical equivalent of an LRA may include an inductor, a resistor,and an alternating current (“AC”) driver. For example, the AC driver,inductor, and resistor may be connected in series as part of a circuit.The LRA's electrical model may act as a low pass filter, thus can bedriven by high frequency driver, such as 25 kHz H-class amplifier

According to some examples, the LRA may be coupled to an amplifier byone or more switches. A microcontroller may provide an input, such as asignal, to the amplifier. When the switches are in a closed position,the signal may be transmitted to the LRA which may cause the LRA tooscillate or vibrate at a certain frequency. When the switches are in anopen position, the signal may not be transmitted to the LRA but the LRAmay still oscillate or vibrate due to the momentum of the mass andmagnet.

When the switches are in an open position a back electromotive force(“EMF”) may be determined. In some examples, an instrumentationamplifier may be coupled to the LRA. The instrumentation amplifier maybe used to determine, or capture, the back EMF when the switches are inthe open position. A drop in back EMF may depend on the amount of touch,the pressure of the touch, and what is touching the device. According tosome examples, when the device is touched, the oscillation or vibrationof the LRA may be dampened. In examples where a human touches thedevice, the human body is able to absorb energy based on the contactforce. The human body may, therefore, absorb some of the energy from theoscillation of the mass of the LRA which may slow, or dampen, theoscillation of the mass of the LRA. As the oscillation of the mass ofthe LRA slows, the value of the back EMF may become lower.

To determine whether the device is being touched, the determined backEMF when the switches are in the open position may be compared to apreviously determined back EMF. A previously determined back EMF may bea back EMF captured 2, 5, 7, 10, 12, etc. readings or cycles ago. Adifference between the determined back EMF and a previously determinedback EMF may be compared to a threshold. In some examples, where thedifference is higher than the threshold it may be determined that thedevice was being touched.

To determine the pressure of a touch on the device, the determined backEMF may be compared to a baseline threshold. For example, once thedevice determines that the device has been touched, the determined backEMF may be subtracted from the baseline threshold. The differencebetween the determined back EMF and the baseline threshold may be theamount of pressure applied to the device.

Using the LRA, which is already in the device, to determine whether thedevice was touched, with how much pressure was the device touched, andwhat surface is touching the device may provide for an increased userexperience. This may provide the device greater sensing capabilities tothe LRA without requiring additional sensors or hardware. In someexamples, using the LRA instead of other sensors and components withinthe device may decrease power consumption. A decrease in powerconsumption may increase the length of time the user may use the devicebefore having to recharge the device. According to some examples, bydetermining whether the device is in contact with the human body or ahard surface, such as a table top, the device may further conservebattery life by not turning the screen on in response to determiningthat it is in contact with a hard surface.

Further, by using the LRA to make determinations about the type ofcontact between the device and its surroundings, the costs andcomplexity of manufacturing the device may decrease. Using the LRA todetermine these types of contact may obviate the need for othercomponents within the device. The LRA may already be within most mobileand wearable devices to provide haptic feedback and, therefore,additional components may not be required during manufacturing.

FIG. 1A illustrates a device 102 in which the features described aboveand herein may be implemented. In this example, device 102, may containone or more processors 104, memory 106, instructions 108, data 110, anamplifier 112, a linear resonant actuator (“LRA”) 114, one or moreswitches 116, an instrumentation amplifier 118, and a microcontroller120.

The one or more processors 104 may be any conventional processors, suchas commercially available microprocessors. Alternatively, the one ormore processors may be a dedicated device such as an applicationspecific integrated circuit (ASIC) or other hardware-based processor.Although FIG. 1A functionally illustrates the processor, memory, andother elements of wearable device 102 as being within the same block, itwill be understood by those of ordinary skill in the art that theprocessor, computing device, or memory may actually include multipleprocessors, computing devices, or memories that may or may not be storedwithin the same physical housing. Similarly, the memory may be a harddrive or other storage media located in a housing different from that ofdevice 102. Accordingly, references to a processor or computing devicewill be understood to include references to a collection of processorsor computing devices or memories that may or may not operate inparallel.

Memory 106 may store information that is accessible by the processors,including instructions 108 that may be executed by the processors 104,and data 110. The memory 106 may be a type of memory operative to storeinformation accessible by the processors 104, including a non-transitorycomputer-readable medium, or other medium that stores data that may beread with the aid of an electronic device, such as a hard-drive, memorycard, read-only memory (“ROM”), random access memory (“RAM”), opticaldisks, as well as other write-capable and read-only memories. Thesubject matter disclosed herein may include different combinations ofthe foregoing, whereby different portions of the instructions 108 anddata 110 are stored on different types of media.

Memory 106 may be retrieved, stored or modified by processors 104 inaccordance with the instructions 108. For instance, although the presentdisclosure is not limited by a particular data structure, the data 110may be stored in computer registers, in a relational database as a tablehaving a plurality of different fields and records, XML documents, orflat files. The data 110 may also be formatted in a computer-readableformat such as, but not limited to, binary values, ASCII or Unicode. Byfurther way of example only, the data 110 may be stored as bitmapscomposed of pixels that are stored in compressed or uncompressed, orvarious image formats (e.g., JPEG), vector-based formats (e.g., SVG) orcomputer instructions for drawing graphics. Moreover, the data 110 maycomprise information sufficient to identify the relevant information,such as numbers, descriptive text, proprietary codes, pointers,references to data stored in other memories (including other networklocations) or information that is used by a function to calculate therelevant data.

The instructions 108 can be any set of instructions to be executeddirectly, such as machine code, or indirectly, such as scripts, by theprocessor 104. In that regard, the terms “instructions,” “application,”“steps,” and “programs” can be used interchangeably herein. Theinstructions can be stored in object code format for direct processingby the processor, or in any other computing device language includingscripts or collections of independent source code modules that areinterpreted on demand or compiled in advance. Functions, methods androutines of the instructions are explained in more detail below.

Device 102 may include a back EMF measurement circuit 122. The back EMFmeasurement circuit 122 may include amplifier 112, linear resonantactuator (“LRA”) 114, one or more switches 116, instrumentation ordifferential amplifier 118, and microcontroller 120. While thecomponents are each represented in FIG. 1A, FIG. 1B illustrates theschematic of the back EMF measurement circuit 122.

The amplifier 112 may be used for input and output of a signal to LRA114. According to some examples, amplifier 112 may be an audioamplifier. Amplifier 112 may receive a signal from microcontroller 120.The amplifier 112 may receive the signal as input and may output thesignal to the LRA 114. The output of amplifier 112 may be in an analogwaveform. The amplifier 112 may include logic and one or more H-bridges124. The H-bridges 124 may be configured to reverse current and enableelectric braking of LRA 114. In some examples, the H-bridge 124 mayoutput a signal that approximates analog waveforms.

As shown in FIG. 1B, amplifier 112 may be coupled to the LRA 114 via oneor more switches 116. According to some examples, switches 116 may beanalog switches. When switches 116 are in the closed position, LRA 114receives the signal transmitted by amplifier 112. The signal transmittedby amplifier 112 to the LRA 114 may be the signal amplifier 112 receivedfrom microcontroller 114. The signal may cause the LRA 114 to oscillate.When switches 116 are in the open position, LRA 114 does not receive thesignal transmitted from the amplifier 112. The back EMF may bedetermined when the switches 116 are in the open position.

The LRA 114 may operate as a linear electric motor and may be driven byalternating current. The LRA 114 may operate at a resonance between 150Hz and 250 Hz. The frequency range of 150 Hz and 250 Hz may correspondto the sensitivity of the tactile receptors of the human body. The LRA114 may be configured to be a resistor-inductor (“RL”) low-pass filter.For example, LRA 114 may convert the signal into an analog waveform.

As shown in FIG. 1C, the LRA 114A may contain a mass 126 attached to aspring 130. A magnet 128 may be coupled to mass 126. The mass 126 andmagnet 128 may move along an axis 134. As the mass 126 and magnet 128oscillates, the mass 126 and magnet 128 may move closer and/or furtheraway from coil 132. Coil 132 may be a voice coil. As the mass 128 andmagnet 128 oscillates relative to coil 132, the magnet 128 may createmagnetic flux. The rate of change of the magnetic flux created by theoscillating magnet 128 may be proportional to the back EMF. For example,as the device is touched, held, worn by a user, or resting on a materialsurface, the oscillation of the mass may dampen based on how muchvibrational energy is absorbed by the object in contact with the device.The dampening of the moving mass may be proportional to the back EMF.

As shown in FIG. 1D, the LRA 114B may be modeled as an electricalcircuit. The circuit may include an AC driver 136, inductor 138,resistor 140, and a back EMF measurement location 142.

When the switches 116 are in the open position, such that LRA 114 doesnot receive the signal, an instrumentation amplifier 118 may be used todetermine the back EMF. The back EMF may be proportional to the rate ofchange of the velocity of the mass of the LRA 114. The back EMF may bemeasured from the floating voltage between the two LRA leads. Forexample, when switches 116 are in the open position, the mass in the LRA114 may still oscillate. The oscillation of the mass within LRA 114 mayproduce oscillating back EMF voltage.

Instrumentation or a differential amplifier 118 may reduce thecommon-mode noise, such as 60 Hz power lines, which may overwhelm thesignal originating from microcontroller 120. Common-mode noise may benoise from outside the device. For example, the noise may be from otherdevices or electrical circuits near the device.

Microcontroller 120 may digitize and process signal. The signal may be,for example, a pulse width modulation (“PWM”). The PWM may have awaveform configured to drive LRA 114. The PWM shape may be configured tooptimize the capture of the back-EMF signal. Additional detailspertaining to the PWM will be discussed herein with respect to FIGS. 2and 3 .

As shown in FIG. 1B, microcontroller 120 may transmit, as output, thesignal to amplifier 112. Microcontroller 120 may receive the back EMF,as input, from instrumentation amplifier 118. The microcontroller 120may transmit the back EMF to the one or more processors 104. The one ormore processors 104, based on the determined back EMF, may determinewhether the device 102 was touched, a pressure of the touch, and what istouching device 102.

FIG. 2 illustrates an example signal transmitted to drive the LRA duringactive sensing. Active sensing may be used when determining whether thedevice was touched and/or the pressure at which the device was touched.Touching or making contact with the LRA while the mass in the LRAoscillates may change the velocity at which the mass oscillates. Forexample, when contact is made with the LRA, the vibrational energy ofthe LRA may be dissipated into the object in contact with the LRA. Insome examples, different amounts of vibrational energy may be absorbedby the object in contact with the LRA based on the amount of pressureapplied to the LRA. The slowing, or dampening, of the oscillation of theLRA may be determined based on the back EMF output by the LRA.

The signal transmitted to drive the LRA may be a PWM waveform 200. Theshape of the waveform may have three equal stages, as indicated by thefiltered PWM output 208. For example, there may be a ramp up stage 202,flat stage 204, and ramp down stage 206. Having three equal stages maycause the mass inside the LRA to move more smoothly and, therefore,decrease the audible crackling noise. The waveform 200 may be optimizedfor sensing. For example, each pulse may be 20 kHz with a width of 1 us.However, the pulse may be 10 kz, 18 kHz, 25 kHz, etc. and the width maybe 0.7 us, 0.87 us, 1.2 us, etc. Therefore, the pulse of 20 kHz andwidth of 1 us is merely one example and is not intended to be limiting.For example, the pulse and the width may be determined based on the LRAon the device.

FIG. 3 illustrates a graph of the acceleration of the LRA superimposedon the output of the back EMF measurement circuit. For example, themeasured acceleration 302 of the LRA may be substantially similar to asine wave. This may represent the periodic motion of the LRA as itoscillates. The output 304 of the back EMF measurement circuit may bemeasured after negative and positive drive half-cycles. The negative andpositive drive half-cycles may be referred to as low and high, as shownon graph 300.

According to some examples, the back EMF may be measured during time306. Time 306 may be when the one or more switches are changed to theopen position such that the LRA does not receive the signal transmittedby the microcontroller and/or amplifier. As shown in graph 300, evenwhen the LRA is not receiving the signal during time 306, the LRA maycontinue to accelerate and, therefore, oscillate. By measuring the backEMF during time 306, external forces applied to the device, and,therefore, the LRA may be measured. The external force may be, forexample, a user's touch, the device being placed on a surface, etc. Theback EMF may be a voltage measurement when the coil, or spring, of theLRA is moving relative to the magnet attached to the mass.

FIG. 4 illustrates example outputs of back EMF measurement circuit 122.For example, graph 400A illustrates an example output when the device isnot being touched, graph 400B illustrates an example where the device isbeing lightly touched, and graph 400C illustrates an example where thedevice is being strongly touched. The touch may be, for example, afingertip touch. A light touch may be when there is less pressure, orforce, applied to the device as compared to the pressure, or force,applied by a strong touch.

Each graph 400A, 400B, 400C may include one or more sampling windows 404and windows 402 when the LRA is receiving the signal. For example, asignal may be output by the microcontroller to the amplifier. Theamplifier may transmit the signal to the LRA until the switches arechanged to the open position. Windows 402 may be times between when theaudio amplifier transmits the signal to the LRA and the time when theswitches are changed to the open position. During time “t” betweenwindows 402, one or more samples may be taken of the LRA. The signalfrom the LRA may be the back EMF.

During time t, the LRA may continue to oscillate. The oscillations ofthe LRA may produce back EMF. When the device is touched during time t,the oscillations may dampen, or lessen, as the energy of theoscillations may be absorbed by whatever object is touching the device.For example, the human body is a viscoelastic material that is capableof absorbing energy. When a user touches the device, the body of theuser may absorb at least some of the energy from the oscillating LRA.This may dampen the oscillation of the LRA which may decrease the backEMF.

Graph 400A illustrates an example where the device is not touched. Asshown, the output of the LRA illustrates that the LRA continued tooscillate. The output of the LRA, corresponding to the back EMF, haspeaks and valleys that correspond to the oscillation of the LRA.

Graph 400B illustrates an example where the device is being touched witha light touch. The oscillations of the LRA in graph 400B during time tare smaller than those in graph 400A. The decrease in back EMF in graph400B may be due to the device being touched. For example, theoscillations of the LRA may have been dampened due to the energy of theLRA being absorbed by the user touching the device. A light touch by theuser may indicate that only some of the energy of the LRA is absorbed bythe user such that there are still some peaks and valleys of back EMFduring time t.

Graph 400C illustrates an example where the device is being touched witha strong touch. The oscillations of the LRA in graph 400C during time tare smaller than those in both graphs 400A and 400B. Similar to graph400B, the decrease in back EMF in graph 400C may be due to the devicebeing touched. A strong touch by the user may indicate the most of theenergy of the LRA is absorbed by the user such that there are almost nopeaks and valley of back EMF during time t. A strong touch may be atouch that absorbs more energy of the LRA than a light touch. Accordingto some examples, a strong touch may be a touch that has more pressurethan a light touch.

To determine the type of contact between the device and itssurroundings, the PWM width may be adjusted to produce minimummeasurable back EMF peaks when the device is touched. The samplingwindow 404 may be calibrated to the position of the highest back EMFpeak in the output of the back EMF measurement circuit. The value of theback EMF during sampling window 404 may be determined. The determinedback EMFs may be stored in the memory of the device. According to someexamples, the determined back EMF may be stored in a first-in-first-out(“FIFO”) buffer such that only a predetermined number of determined backEMF values are stored. To determine whether the device was touched, themost recent determined back EMF may be compared to a previouslydetermined back EMF value. The previously determined back EMF value maybe one of the predetermined number of determined back EMF values storedin the FIFO buffer. If the difference between the most recentlydetermined back EMF value and the previously determined back EMF valueis greater than a predetermined threshold, the device was touched. Thepredetermined threshold may be, for example, 65 mV. According to someexamples, the predetermined threshold may be dependent on the deviceand/or the LRA such that the predetermined threshold may be 42 mV, 57mv, 71 mV, etc. Thus, the predetermined threshold of 65 mV is merely oneexample and is not intended to be limiting.

To determine the pressure of contact with the device, the most recentdetermined back EMF may be compared to a baseline threshold. Forexample, the difference between the baseline threshold and the mostrecent determined back EMF may correspond to the pressure of the touch.

FIG. 5 illustrates an example of mean sampling of low and high cyclesover time. For example, graphs 500A and 500B illustrate the output 502A,502B of the back EMF measurement circuit during low and high cycleswhile the device is being touched. The device may be touched during time504. As shown in graphs 500A, 500B, the back EMF may remain constant butfor when the device is being touched. As shown in graph 500A, when thedevice is touched during time 504 the high back EMF decreases. Thedecrease in high back EMF may be due to at least some of the energy ofthe LRA being absorbed by the object touching the device. As shown ingraph 500B, when the device is touched during time 504 the low back EMFincreases.

FIG. 6 illustrates an example sensing circuit to determine whether thedevice has been tapped. For example, tap detection may be monitored atlow power and may be used as a wake-up gesture for the device. Tapdetection may not use active sensing and, therefore, does not need toactuate the LRA by transmitting a signal. When the device is tapped, themagnet of the LRA may move relative to the coil. Sensing circuit 600 maybe used to detect the tap. Sensing circuit may include a comparator 602coupled to the LRA 614. According to some examples, the comparator 602may have a threshold of 1.125V. However, the threshold of comparator 602may be more or less than 1.125V, such as 0.8V, 1.02V, 1.132V, etc. Thus,the threshold of 1.125V is merely one example and is not intended to belimiting.

Comparator 602 may include a microcontroller. The microcontroller maydetect interrupts produced by comparator 602 while in sleep mode. Forexample, microcontroller 602 may detect when the device is tapped basedon the back EMF output by the LRA 614.

FIG. 7 illustrates an example back EMF produced by tapping the device.Graph 700 includes comparator trigger output 702, back EMF output 704,and threshold 706. The back EMF output 704 may correspond to the backEMF output by LRA 614. For example, as the device is tapped, the magnetwithin the LRA may move relative to the coil. The movement of the magnetrelative to the coil may produce a back EMF. The back EMF output 704 maybe compared to the threshold 706 to determine whether the device wastapped. The threshold voltage 706 may correspond to the threshold ofcomparator 702. The comparator trigger output 702 may correspond to anaction of the device in response to determining the device was tapped.For example, if it is determined that the device was tapped, thecomparator trigger output 702 may indicate that the screen of the deviceturns on.

According to some examples, the passive LRA sensing, discussed withrespect to FIGS. 6 and 7 , may be combined together with the active LRAsensing, discussed with respect to FIGS. 1B-5 . The passive and activeLRA sensing may be combined with one or more switches. In some examples,the passive and active LRA sensing may be implemented separately forpower optimization.

The LRA may be used as combined input/output devices in portableelectronics. Portable electronics may include mobile and wearabledevices. For example, an LRA may be coupled to the portable electronicas a button to provide tap, touch, and pressure sensing. The LRAs mayadditionally provide haptic feedback once touch is detected.

FIGS. 8A and 8B illustrate example feedback based on the coupling of awearable device to the skin of a user. According to some examples, thewearable device may be a smartwatch. The wearable device may include ashort range communications interface that allows the accessory to becoupled to the host device. The wireless coupling may be establishedusing any of a variety of techniques, such as Bluetooth, Bluetooth lowenergy (BLE), etc. The host device may be, for example, smartphone, alaptop computer, or any of a variety of other host devices, such asgaming systems, tablets, etc.

For the wearable device to provide haptic feedback with consistentforce, the wearable device may be coupled to the skin such that the LRAapplies constant contact pressure to the skin of the user at apredetermined target pressure. Constant contact pressure may be thepressure between the device and the skin of the user that remains steadfor more than a threshold period of time. The LRA may be used todetermine an amount of constant contact pressure between the user andthe wearable device. For example, the band of the smartwatch may not betoo tight or too loose.

As shown in FIG. 8A, if the LRA of smartwatch 802 determines that theconstant contact pressure between the user 804 and smartwatch 802 is tootight, smartwatch 802 may provide a notification 808A, 808B to loosenthe watchband. The notification 808A, 808B may be provided on thedisplay of smartwatch 802 and/or the display of host device 806.

As shown in FIG. 8B, if the LRA of smartwatch 802 determines that theconstant contact pressure between the skin of the user 804 andsmartwatch 802 is too loose, smartwatch 802 may provide a notification808C, 808D to tighten the watchband. The notification 808C, 808D, may beprovided on the display of smartwatch 802 and/or the display of hostdevice 806.

In some examples, if the wearable device determines, based on thedetermined amount of constant contact pressure between the user and thewearable device, that the amount of constant contact pressure is tootight or too loose, the wearable device may adjust the amplitude of thevibrotactile force to compensate for the tightness or looseness. Forexample, if the determined constant contact pressure is too tight, theamplitude of the vibrotactile force may decrease. In some examples, ifthe determined constant contact pressure is too loose, the amplitude ofthe vibrotactile force may increase.

FIGS. 9A-9C illustrate example notifications based on the sensedenvironment. The LRA on the device may adjust the haptic and/or audiofeedback based on the sensed environment. For example, the device maydetermine that material surface the device is resting on based on howmuch vibrational energy is dissipated. The vibrational energy maydissipate more or less depending on what surface or object is touchingthe device. For example, the vibrational energy may dissipate more whenthe device is being held by a user than when the device is placed on ahard surface, such as a table. In some examples, the determined back EMFmay change based on how much vibrational energy is absorbed by thesurface. The softer the surface, the more vibrational energy that isabsorbed such that the oscillation of the LRA may dampen more. As theoscillation of the LRA is dampened, the amount of back EMF may decreaseas compared to when the device is on a hard surface. In examples wherethe device is on a hard surface, less vibrational energy may be absorbedby the surface such that the oscillation of the LRA dampens less thanwhen the device is on a soft surface. Machine learning models may beused to classify and determine the material surface in contact thedevice. Based on the material surface in contact with the device, thedevice may adjust the amplitude of the vibrotactile force. A change inthe vibrotactile force may change the intensity of the haptic feedbackof the device. For example, if the amplitude of the vibrotactile forceis increased, the intensity of the haptic feedback may be increased. Ifthe vibrotactile force is decreased, the intensity of the hapticfeedback may be decreased.

As shown in FIG. 9A, the device 906 may be on a soft surface 910, suchas foam, a couch, a bed, etc. A soft surface 910 may be a surface thatallows for the vibrational energy of the device to dissipate as the softsurface may absorb at least some of the vibrational energy of thedevice. The device 906 may determine it is on a soft surface 910 basedon how much energy is being absorbed by the surface. According to someexamples, the device 906 may determine it is on a soft surface 910 basedon the determined back EMF. Once the device 906 determines it is on asoft surface 910, the device 906 may provide a notification 908Aindicating an adjustment to the vibrotactile force and/or audio outputfor notifications. For example, haptic feedback may not be felt and/orheard when the device is on a soft surface. According to some example,if the device 906 determines it is on a soft surface, the device 906 maydecrease the vibrotactile force and increase the audio output fornotifications. A notification 908A may be provided on the deviceindicating these changes.

As shown in FIG. 9B, the device 906 may be held by a user 904. The humanbody is a viscoelastic matter that can absorb the vibrational energy ofthe device. As the user's body absorbs the vibrational energy of thedevice, the device may determine a back EMF. The device 906 maydetermine a length of time that the vibrational energy is beingabsorbed. The length of time that the vibrational energy is beingabsorbed may indicate that the user tapped, touched, or is holdingdevice 906. For example, a tap may have a duration of zero (0) to 0.5seconds while a touch may have a duration greater than 0.5 seconds.However, a tap may be greater than 0.5 and a touch may be less 0.5seconds. Thus, the example ranges provided for a tap and a touch aremerely one example and is not intended to be limiting. The device 906may determine it is being held when the duration is longer than theduration of a touch.

Based on the determined back EMF and the duration of the contact withdevice 906, the device 906 may determine that it is being held by theuser 904. Once the device 906 determines that it is being held by theuser 904, the device 906 may provide a notification 908B indicating anadjustment to the vibrotactile force and/or audio output fornotifications. For example, haptic and/or audio feedback may not berequired for notifications as the user is currently holding the device.In such an example, the device 906 may turn off haptic feedback andaudio output for notifications. A notification 908B may be provided onthe device indicating these changes.

As shown in FIG. 9C, the device 906 may be on a hard surface 912, suchas a wood or glass table. As the hard surface 912 will not absorb thevibrational energy of the device 906, the vibrational energy of thedevice 906 will not dissipate as it would on the soft surface asdescribed above. The device 906 may determine it is on a hard surface912 based on how much and/or little of the vibrational energy of thedevice is absorbed by the surface. After device 906 determines that isit on a hard surface 912, the device 906 may provide a notification 908Cindicating an adjustment to the vibrotactile force and/or audio outputfor notifications. For example, if the device 906 was only set toprovide haptic feedback for a notification when the device 906 is putonto a hard surface, the device 906 may adjust the feedback to be bothhaptic and audio feedback. The notification 908C may indicate that bothvibration and audio feedback is turned on.

FIG. 10 illustrates an example method of determining a type of contactwith a device. The following operations do not have to be performed inthe precise order described below. Rather, various operations can behandled in a different order or simultaneously, and operations may beadded or omitted.

For example, in block 1010 a signal may be transmitted to a LRA by amicrocontroller in communication with the LRA. In some examples, thesignal may be transmitted to an amplifier by the microcontroller incommunication with the LRA and the amplifier. The signal may be a PWMwaveform. The PWM waveform may be optimized for sensing. For example,the waveform may have three equal stages including a ramp up stage, aflat stage, and a ramp down stage. Splitting the waveform into equalstages may cause the mass inside the LRA to move more smoothly such thatthe audible crackling noise may be decreased.

According to some examples, the amplifier may be coupled to the LRA byone or more switches. The one or more switches may be switched, by themicrocontroller, from a closed position to an open position or from theopen position to the closed position. According to some examples, whenthe one or more switches are in the open position the signal may not betransmitted from the amplifier to the LRA.

In block 1020, the microcontroller may measure, or determine, a back EMFof the LRA. For example, based on the type of contact with the device,the oscillation of the LRA may dampen. The dampening of the oscillationof the LRA may change the back EMF produced by the LRA. Determining theback EMF produced by the LRA as the oscillation dampens may be used todetermine the type of contact made with the device.

According to some examples, the microcontroller may measure the back EMFof the LRA when the one or more switches are in the open position. Forexample, when the one or more switches are in the open position, the LRAmay continue to oscillate. The back EMF may be measured as the LRAcontinues to oscillate without being driven by a signal. According tosome examples, the back EMF may be captured and/or measured using adifferential amplifier, such as an instrumentation amplifier.

In block 1030, the microcontroller may transmit, to one or moreprocessors, the measured back EMF.

In block 1040, the one or more processors may determine, based on themeasured back EMF, a type of contact with the device. The type ofcontact may include a user holding the device, the device resting on amaterial surface, the device being worn by a user, or a touch. The typeof contact between the device and its surroundings may be based on atleast one of a speed of contact with the device, a pressure of contactwith the device, a duration of contact with the device, a material incontact with the device, or a surface area in contact with the device.For example, to determine if the type of contact is a touch, themeasured back EMF may be compared to a previously measured back EMF. Ifthe difference between the measured back EMF and the previously measuredback EMF is greater than a predetermined threshold, the device maydetermine that the type of contact is a touch. To determine the pressureof contact with the device, the measured back EMF may be compared to abaseline threshold. The difference between the measured back EMF and thebaseline threshold may correspond to the pressure of the contact.

Unless otherwise stated, the foregoing alternative examples are notmutually exclusive, but may be implemented in various combinations toachieve unique advantages. As these and other variations andcombinations of the features discussed above can be utilized withoutdeparting from the subject matter defined by the claims, the foregoingdescription of the embodiments should be taken by way of illustrationrather than by way of limitation of the subject matter defined by theclaims. In addition, the provision of the examples described herein, aswell as clauses phrased as “such as,” “including” and the like, shouldnot be interpreted as limiting the subject matter of the claims to thespecific examples; rather, the examples are intended to illustrate onlyone of many possible embodiments. Further, the same reference numbers indifferent drawings can identify the same or similar elements.

The invention claimed is:
 1. A wearable device, comprising: a bandconfigured to secure the device to a skin surface of a user; a resonantactuator; and one or more processors in communication with the resonantactuator, the one or more processors configured to: measure a backelectromotive force (“EMF”) of the resonant actuator; determine, basedon the measured back EMF, information representative of a contactpressure between the device and the skin surface of the user, thecontact pressure being greater when the band is tighter, the contactpressure being lesser when the band is looser; and output, based atleast in part on the determined information representative of thecontact pressure, a notification relating to a tightness of the band. 2.The wearable device of claim 1, wherein the one or more processors arefurther configured to compare the determined information representativeof the contact pressure with a predetermined target pressure.
 3. Thewearable device of claim 2, wherein: when the determined informationrepresentative of the contact pressure is greater than the predeterminedtarget pressure, the notification is a notification to loosen thedevice, and when the determined information representative of thecontact pressure is less than the predetermined target pressure, thenotification is a notification to tighten the device.
 4. The wearabledevice of claim 1, wherein the resonant actuator is a linear resonantactuator.
 5. The wearable device of claim 1, wherein the notification isa notification to tighten or loosen the band.
 6. The wearable device ofclaim 1, wherein the one or more processors are further configured toadjust, based on the determined information representative of thecontact pressure, a vibrotactile force.
 7. The wearable device of claim1, wherein the one or more processors are further configured todetermine, based on the measured back EMF and at least one of a speed ofcontact with the wearable device, a pressure of contact with thewearable device, a duration of contact with the wearable device, anelasticity of a material in contact with the wearable device, or asurface area in contact with the wearable device, a type of contactbetween the wearable device and its surroundings.
 8. The wearable deviceof claim 7, wherein the type of contact includes the user holding thewearable device, the wearable device resting on a material surface, thewearable device being worn by the user, or a touch.
 9. A method,comprising: measuring, by one or more processors, a back electromotiveforce (“EMF”) of a resonant actuator; and determining, by the one ormore processors based on the measured back EMF, informationrepresentative of a contact pressure between a wearable device and askin surface of a user, the wearable device including a band configuredto secure the wearable device to the skin surface of the user, thecontact pressure being greater when the band is tighter, the contactpressure being lesser when the band is looser; and output, based atleast in part on the determined information representative of thecontact pressure, a notification relating to a tightness of the band.10. The method of claim 9, further comprising comparing, by the one ormore processors, the determined information representative of thecontact pressure with a predetermined target pressure.
 11. The method ofclaim 10, wherein: when the determined information representative of thecontact pressure is greater than the predetermined target pressureoutputting the notification further comprises outputting, by the one ormore processors, a notification to loosen the wearable device, and whenthe determined information representative of the contact pressure isless than the predetermined target pressure outputting the notificationfurther comprises outputting, by the one or more processors, anotification to tighten the wearable device.
 12. The method of claim 9,wherein the resonant actuator is a linear resonant actuator.
 13. Themethod of claim 9, wherein outputting the notification includesoutputting, by the one or more processors, a notification to tighten orloosen the band of the wearable device.
 14. The method of claim 9,further comprising, adjusting, by the one or more processors based onthe determined information representative of the pressure, avibrotactile force.
 15. The method of claim 10, further comprisingdetermining, by the one or more processors based on the measured backEMF and at least one of a speed of contact with the wearable device, apressure of contact with the wearable device, a duration of contact withthe wearable device, an elasticity of a material in contact with thedevice, or a surface area in contact with the wearable device, a type ofcontact between the wearable device and its surroundings.
 16. The methodof claim 15, wherein the type of contact includes the user holding thewearable device, the wearable device resting on a material surface, thewearable device being worn by the user, or a touch.
 17. A wearabledevice, comprising: a resonant actuator; and one or more processors incommunication with the resonant actuator, the one or more processorsconfigured to: measure a back electromotive force (“EMF”) determine,based on the measured back EMF, a contact pressure between the wearabledevice and a skin surface of a user; and output, based at least in parton the determined contact pressure, a notification to tighten or loosenthe wearable device.
 18. The wearable device of claim 17, wherein theone or more processors are further configured to compare the determinedcontact pressure and a predetermined target pressure.
 19. The wearabledevice of claim 18, wherein: when the determined contact pressure isgreater than the predetermined target pressure, the notification is anotification to loosen the wearable device, and when the determinedcontact pressure is less than the predetermined target pressure, thenotification is a notification to tighten the wearable device.
 20. Thewearable device of claim 17, wherein the device is a wearable device.